1. Field of the Invention
The present invention relates to an optical amplifier and optical amplification method for amplifying wavelength division multiplexed signal light. In particular, the invention relates to an optical amplifier and optical amplification method for collectively amplifying wavelength division multiplexed signal light which includes optical signals of two wavelength bands.
2. Description of the Related Art
With conventional long distance optical transmission systems, optical transmission is performed using optical regenerating repeaters which convert optical signals into electric signals to effect re-timing, re-shaping, and regenerating. However recently, with progress in the utilization of optical amplifiers, optical amplifying-and-repeating transmission systems which use optical amplifiers as linear repeaters are being investigated. By replacing an optical regenerating repeater with an optical amplifying repeater, the number of parts in the repeater can be greatly reduced, with the prospect of maintaining reliability and greatly reducing costs.
Furthermore, as one method of realizing a large capacity of an optical transmission system, a wavelength division multiplexed (WDM) optical transmission system which multiplexes two or more optical signals having different wavelengths to transmit on a single optical transmission path is attracting attention. In a WDM optical amplifying repeater transmission system with the aforementioned optical amplifying-and-repeating transmission system combined with a WDM optical transmission system, it is possible to collectively amplify WDM signal light using an optical amplifier, thus enabling the realization of large capacity and long distance transmission with a simple (economic) configuration.
For the wavelength band of the WDM signal light used in the aforementioned conventional optical transmission system, in addition to a so called C band of 1550 nm band, recently a so called L band of 1580 nm band is being investigated. Moreover, a WDM optical amplifying-and-repeating transmission system where both are transmitted along a single transmission path (a C/L band WDM optical amplifying-and-repeating transmission system) is being investigated.
As an optical amplifier having an optical amplification band in both the C band and the L band, there is known for example an article xe2x80x9cRecent Research Direction Related to Optical Fiber Amplifiers and Wide Bandsxe2x80x9d by Yamada et al, Technical Report of IEICE. OCS 97-42, ED 97-132 OPE 97-87 LQE 97-87 (1997-11). This optical amplifier, as shown in the block diagram of FIG. 49, is of a simple parallel configuration where WDM signal light to be input thereto is demultiplexed by a demultiplexer into a C band and an L band, and after being respectively amplified in a C band optical amplifying section and an L band optical amplifying section, is multiplexed by a multiplexer and then output.
However, since the multiplexer is provided on the output side of the respective C band and the L band optical amplifying sections, there is a problem that the noise figure deteriorates by the insertion loss component. As a technique for coping with this, there is known for example an optical amplifier disclosed for example in xe2x80x9cOptical Amplifiers and their Applications, Post deadline 2 of ""97 published by Lucent. A schematic configuration of this is shown in the block diagram of FIG. 50. With the configuration of FIG. 50, a C/L band optical amplifying section having an amplifying band in both the C band and the L band is arranged before the demultiplexer in the configuration of FIG. 49. Hence at least a gain equal to or greater than the insertion loss component of the demultiplexer is ensured by the C/L band optical amplifying section, so that an improvement in the noise figure is achieved.
However, with the aforementioned conventional C/L band optical amplifier, in the case of the configuration shown in FIG. 50, since this has three or more optical fiber amplifying sections, the number of excitation light sources also increases. As a result, the power consumption of the overall optical amplifier increases. In particular, when as with optical amplifiers used for example as undersea repeaters or the like, there are restrictions from the view point of installation space, power consumption or other factors, it is difficult to realize a configuration such as shown in FIG. 50.
Furthermore, a system where the respective signal lights of the C/L band are transmitted in bi-directions of the ascending line and the descending line is also being investigated, and hence there is a strong demand for a C/L band optical amplifier of a simple configuration, which copes with even for such a system.
The present invention focuses on the above points, with an object of providing, for an optical amplifier and an optical amplification method which performs amplification of optical signals of two wavelength bands, an optical amplifier and an optical amplification method of a simple configuration which can deal with restrictions on installation space, power consumption and the like.
An optical amplifier according to the present invention for achieving the aforementioned object, as shown in FIG. 1, is constituted so that in an optical amplifier for amplifying wavelength division multiplexed signal light which contains respective optical signals of a first wavelength band and a second wavelength band, there is provided an optical amplifying means 1 for amplifying the wavelength division multiplexed signal light using a rare earth element doped fiber to which excitation light is supplied, a first optical circulator 2A having at least three ports, connected to one end of the rare earth element doped fiber, a second optical circulator 2B having at least three ports, connected to the other end of the rare earth element doped fiber, wherein an optical signal of the first wavelength band is input to the rare earth element doped fiber via the first optical circulator 2A and output from the rare earth element doped fiber via the second optical circulator 2B, and an optical signal of the second wavelength band is input to the rare earth element doped fiber via the second optical circulator 2B, and output from the rare earth element doped fiber via the first optical circulator 2A so that signal light of the respective wavelength bands is propagated in mutually opposite directions inside the rare earth element doped fiber.
With such a configuration, the respective signal lights of the first wavelength band and the second wavelength band are input and output to the rare earth element doped fiber via the respective optical circulators 2A and 2B so as to have mutually opposite propagation directions, then even with an optical amplifier which uses only one rare earth element doped fiber, the optical signals of the respective wavelength bands are collectively amplified. Moreover, the respective optical isolators 2A and 2B achieve a function equivalent to an optical isolator or the like conventionally used in order to prevent the influence of reflection light. As a result, the configuration of the optical amplifier can be simplified, and low power consumption achieved.
A specific configuration for the aforementioned optical amplifier may be such that the first optical circulator 2A outputs optical signals of the first wavelength band input to a first port P1 from a second port P2 connected to one end of the rare earth element doped fiber, and outputs optical signals of the second wavelength band which are propagated inside the rare earth element doped fiber and input to the second port P2 from a third port P3, and the second optical circulator 2B outputs optical signals of the second wavelength band input to a first port P1 from a second port P2 connected to the other end of the rare earth element doped fiber, and outputs optical signals of the first wavelength band which are propagated inside the rare earth element doped fiber and input to the second port P2 from a third port P3.
Moreover, the wavelength division multiplexed signal light may be specifically set such that the first wavelength band is a 1580 nm band and the second wavelength band is a 1550 nm band.
For the aforementioned optical amplifier, in the case where the wavelength band is set as above, then as shown in FIG. 2, the configuration may be such that there is provided an auxiliary optical amplifying means 1xe2x80x2 having an optical amplifying band in the first wavelength band, for amplifying only optical signals of the first wavelength band output from the second optical circulator 2B to output.
With such a configuration, even in the case where optical amplification with respect to the first wavelength band in the optical amplifying means 1 is insufficient compared to that for the second wavelength band, this insufficiency is amplified by the auxiliary optical amplifying means 1xe2x80x2, enabling optical signals of a substantially uniform level to be obtained for the respective wavelength bands.
A specific configuration for the aforementioned optical amplifier may be such that, at the time of amplifying wavelength division multiplexed signal light which contains respective optical signals of the first wavelength band and the second wavelength band transmitted in the same direction, there is provided a demultiplexing means for demultiplexing the wavelength division multiplexed signal light into respective signal lights of the first wavelength band and the second wavelength band, and outputting the demultiplexed signal light of the first wavelength band to a first port P1 of the first optical circulator 2A, and outputting the signal light of the second wavelength band to a first port P1 of the second optical circulator 2B, and a multiplexing means for multiplexing the optical signal of the second wavelength band output from the third port P3 of the first optical circulator 2A and the optical signal of the first wavelength band output from the third port P3 of the second optical circulator 2B.
Moreover, the configuration may be such that at the time of amplifying wavelength division multiplexed signal light which contains respective optical signals of the first wavelength band and the second wavelength band transmitted in opposite directions, there is provided first and second optical multiplexing/demultiplexing means respectively provided with a function for demultiplexing the wavelength division multiplexed signal light into respective signal lights of the first wavelength band and the second wavelength band, and for multiplexing the respective signal lights of the first wavelength band and the second wavelength band, and the first optical multiplexing/demultiplexing means demultiplexes the signal light of the first wavelength band to send to a first port P1 of the first optical circulator 2A, and multiplexes the signal light of the second wavelength band sent from a third port P3 of the first optical circulator 2A and outputs the multiplexed signal light, and the second optical multiplexing/demultiplexing means demultiplexes the signal light of the second wavelength band to send to a first port P1 of the second optical circulator 2B, and multiplexes the signal light of the first wavelength band sent from a third port P3 of the second optical circulator 2B and outputs the multiplexed signal light.
Other objects, characteristics and advantages of the present invention will become apparent from the following description of embodiments given in conjunction with the appended drawings.